Methionine can be biosynthesized by microorganisms and plants, although mammals cannot biosynthesize methionone de novo. Methionine is required for many important cellular functions. It is a basic building block of proteins and is required for the initiation of protein synthesis (via N-formyl-L- methionine). Methionine is also used in the synthesis of S-adenosyl-L-methionine which is the major methyl group donor in cellular metabolism. S-adenosyl-L-methionine is also utilized in pathways such as autoinducer AI-2 biosynthesis I and spermidine biosynthesis I.

Some bacteria including Escherichia coli synthesize methionine using organic sulfur through transsulfuration of O-acylated homoserine with cysteine to form cystathionine. Cystathionine is then cleaved to homocysteine, and methylated to methionine as shown in this pathway [Soda87]. Other bacteria, yeast and fungi can use a different route in which they directly assimilate inorganic sulfur by a sulfhydrylation (see MetaCyc pathways sulfate reduction I (assimilatory), L-homocysteine biosynthesis and L-methionine biosynthesis III). While many organisms seem to contain both routes for methionine biosynthesis, in E. coli and other enteric bacteria only the transsulfuration pathway is used. Reviewed in Greene, R.C. (1996) "Biosynthesis of Methionine" in [Neidhardt87] pp. 542-560).

The O-acyl group of homoserine is a succinyl group in enteric bacteria and some other Gram-negative bacteria, such as Pseudomanas aeruginosa and Pseudomonas putida. It is an acetyl group in fungi, yeast, and most Gram-positive bacteria ([Vermeij99, Soda87, Thomas97]). In both routes homocysteine is methylated to methionine via either a cobalamin-independent enzyme, or a cobalamin-dependent enzyme, depending upon the species or growth conditions [Ruckert03, Thomas97].

Homoserine is first activated by O-succinylation in a reaction catalyzed by MetA. The product O-succinyl-L-homoserine combines with cysteine to form cystathionine in a reaction catalyzed by MetB. Lyase cleavage of cystathionine by MetC forms homocysteine. This β-cystathionase activity can also be supplied by MalY as demonstrated in vivo by the ability of constitutive MalY expression to complement metC mutants auxotrophic for methionine [Zdych95]. Homocysteine is subsequently methylated by either MetH or MetE to produce methionine. In E. coli MetH can function only in the presence of exogenously supplied vitamin B12 (cobalamin), which represses MetE expression. B12 is likely to be available in the gut. In the absence of exogenously supplied B12, MetE catalyzes this final step of de novo methionine biosynthesis. Although the pathway is largely regulated at the transcriptional level, feedback inhibition of the first enzyme homoserine O-succinyltransferase (MetA) by methionine and S-adenosyl-L-methionine has been shown.

Under stressful conditions there is further regulation of the pathway enzymes. Under heat-shock conditions growth is slowed due to the thermal instability of MetA. Oxidative stress affects MetE which contains an oxidation-sensitive cysteine residue at position 645 near the active site. Oxidation of methionone itself can also occur although the cell contains methionine sufloxide reductases MsrA and MsrB to combat this. Weak organic acids also generate oxidative stress, with more complex effects. Sulfur limitation depletes homocysteine which serves as a coactivator for MetR activation of MetE expression.

Due to the absence of this pathway in mammals, some of the bacterial biosynthetic enzymes are potential drug targets [Ejim04]. In addition, although methionine is used as a food additive and a medication, its industrial scale production in microorganisms has not yet been achieved due to the complexity and strong regulation of its biosynthetic pathway [Usuda05].